Voltage regulators provide a fixed output voltage regardless of a certain amount of variation in a supply voltage. They may be used to yield more robust electronic systems that include digital circuits, such as microprocessors, as well as mixed signal analog/digital circuits.
A conventional voltage regulator is shown in FIG. 8. A reference circuit (not shown) provides a bandgap reference voltage Vin which is temperature compensated and fixed at 1.25V regardless of fluctuations in the supply voltage Vdd. The reference voltage Vin is then amplified to a higher voltage at node ND12, and then buffered by a second stage including operational amplifier (opamp) 812, drive transistor M28, and load transistor M32, to provide greater drive capability at the output node ND14. This conventional design, however, suffers in several areas, particularly when implemented in a MOS process. First, to achieve low power consumption, the R1-R2 resistor network used to obtain voltage gain in the first stage having opamp 808 requires large value resistors in the range of hundreds of thousands of Ohms. Such large value resistors, however, consume a relatively large and valuable area on a MOS chip, particularly when implemented using polysilicon or source/drain diffusions. Although a "well" structure, formed in a substrate of an integrated circuit, for building the resistors consumes a smaller area than a polysilicon structure, for the same resistance value, the resistance obtained from the well structure is highly voltage dependent. Such voltage dependence precludes an accurate, regulated output voltage in a large number of mass produced chips, because the chips typically display manufacturing variations that cause significant voltage differences between them. Another drawback of the conventional well-based resistor design is its poor supply noise rejection. Using well structures for the R1-R2 resistor network in the first opamp stage allows noise in the substrate of the chip, where the substrate is at Vss, typically ground, to easily enter the signal path of the regulated output voltage, be amplified and appear in the output voltage.